Gas sensor based on protonic conductive membranes

ABSTRACT

A low cost room temperature electrochemical gas sensor for sensing CO and other toxic analyte gases has a solid protonic conductive membrane with a low bulk ionic resistance. A sensing electrode and a count counter electrode, which are separated by the membrane, can be made of mixed protonic-electronic conductors. Embodiments of the inventive sensor also include an electrochemical analyte gas pump to transport the analyte gas away from the counter electrode side of the sensor. Analyte gas pumps for the inventive sensor include dual pumping electrodes situated on opposite sides of the membrane, and include a means for applying a DC power across the membrane to the sensing and counter electrodes. Another embodiment of the inventive sensor has first and second solid protonic conductive membranes, one of which has a sensing electrode and a counter electrode separated by the first membrane, and the other of which has dual pumping electrodes situated on opposite sides of the second membrane.

FIELD OF THE INVENTION

The invention relates to electrochemical gas sensors, and particularlyrelates to electrochemical gas sensors having a sensing electrode, acounter reference electrode, and a solid proton conductor for roomtemperature detection of the concentration of carbon monoxide (CO) inthe ambient.

BACKGROUND OF THE INVENTION

In most prior art solid state commercial gas sensors, it is necessary toheat the sensor element to elevated temperatures in order to acquireboth fast response time and high sensitivity to objective gases. Forexample, N-type semiconductor tin oxide gas sensors and catalyticcombustion type Pd/Pt gas sensors must usually be operated in atemperature range of ca. 200° to 500° C. These sensors must be equippedwith heaters connected to external power sources. Therefore, roomtemperature CO gas sensors, which use less power, are desirable.

It is well known that CO reacts with moisture in air at roomtemperature, and forms protons, electrons, and CO2 in an oxidationreaction of CO.CO+H₂O→CO₂+2H⁺+2e⁻  (1)

It is also known that there is a moisture formation reaction bycombining protons, electrons, and oxygen in a reduction reaction ofoxygen:2H⁺+2e⁻+½O₂→H₂O  (2)

These two reactions are the basis of prior art room temperature lowpower electrochemical gas sensors utilizing a proton conductor. FIG. 1shows the transport processes of such a CO gas sensor. A protonicconductor 12 conducts ionized hydrogen atoms from a sensing electrode 16where the sensor signal originates from the oxidation reaction of carbonmonoxide at sensing electrode 16. Ionized hydrogen atoms, each of whichconstitutes a single proton, are conducted through protonic conductor 12to a counter electrode 14. Electrons that are liberated in the oxidationof carbon monoxide at sensing electrode 16 are conducted through anelectrical lead 22 to voltage meter 18, through an electrical lead 20,and to counter electrode 14 for a reduction reaction of oxygen. In asteady state reaction, the hydrogen ions are transported from sensingelectrode 16 to counter electrode 14 in the depicted potentiometric COgas sensor.

The current generated by the reactions depicted in FIG. 1 can also bemeasured by an amp meter 24 having a resistor R_(L) 26, which circuitrepresents a transport process of an amperometric CO sensor. Absent ampmeter 24, resistor R_(L) 26, the leads thereto which are shown inphantom, transport processes of a potentiometric CO gas sensor are shownfor voltage meter 18 and leads 20, 22.

Whether the transport processes shown in FIG. 1 are for potentiometricCO gas sensor or for an amperometric CO sensor, electrons from theprocess of the oxidation reaction of carbon monoxide travel as seen inarrow 21 in FIG. 1 through leads 20, 22.

The sensor of FIG. 1 is operated in a current mode when the sensing andcounter electrodes 16, 14 are connected to each other through loadresistor R_(L), or are connected to a DC power source (not shown) whichelectrically drives the protons across proton conductor 12.

A prior art room temperature proton conductor sensor developed byGeneral Electric using a polymer porous support material saturated by aliquid proton conductor, has been constructed as an electrochemicalamperometric CO gas sensor (the G. E. Sensor). In the G. E. Sensor, aliquid reservoir was used to provide the liquid proton conductor to theporous support material. Protons, which are indicative of the ambient COconcentration, were driven across the porous support material throughthe liquid conductor by a DC voltage. Electrical current response of thesensor to ambient CO concentration was linear. The cost of the sensorwith such a complicated design, however, is high and is thus not besuitable for practical consumer applications.

In U.S. Pat. No. 4,587,003, a room temperature CO gas sensor using aliquid proton conductor is taught. Basically, the mechanism and designof the sensor were similar to the G. E. sensor, except that the outsidesurfaces of the sensing and counter electrodes of the sensor in thispatent were coated by porous NAFION™ layers. The CO room temperature gassensor taught in the patent currently costs about $200.00. The lifetimeof such a sensor is about 6-12 months due to the rapid drying of theliquid of the electrolytes. In addition, the sensor requires maintenancedue to leakage and corrosion of liquid electrolyte.

The discovery of room temperature solid proton conductors arousedconsiderable efforts to investigate low cost, all-solid electrochemicalroom temperature CO gas sensors. One such sensor that was developed wasa room temperature CO gas sensor with a tubular design using protonconductors, electronically conductive platinum or the like as thesensing electrode, and electronically conductive silver, gold, graphiteor the like as the counter electrode. The sensing electrode decomposedcarbon monoxide gas to produce protons and electrons, whereas thecounter electrode exhibited no activity to decompose carbon monoxidewith the result that a Nemst potential occurred between the twoelectrodes. Thus, carbon monoxide gas was detected.

In detecting carbon monoxide with the tubular design sensor, protons andelectrons are generated at the sensing electrode. For the reaction to becontinued, protons and electrons must be removed from the reactionsites, and CO and moisture must be continuously provided from thegaseous phase to the reaction sites. Therefore, the CO reaction onlyoccurs at three-phase contact areas. The three-phase contact areasconsist of the proton membrane phase, the platinum electron phase, andthe gas phase. Due to the limited three-phase contact areas in thetubular design sensor, the CO reaction was slow. Additionally, theresponse signal was weak. Further, the Nernst potential was not zero inclean air.

A modified electrochemical CO room temperature gas sensor using a planaror tubular sensor design was a subsequent development to the earliertubular design CO sensor. In order to overcome the problem that theNernst potential is not zero in clean air experienced with the earliertubular design CO sensor, the improved design proposed a four probemeasurement method for CO gas detection. The improved design achieved azero reading in clean air, and the improved sensor was insensitive tovariations in relative humility. Theoretical analysis based onelectrochemistry, however, indicates that there is no difference betweenthe four probe method and the normal two probe method of the earliertubular design CO sensor. The improved sensor still used electronicconductors for both the sensing and counter electrodes, and showed slowand weak response signals to CO gas.

A still further improved design of a CO sensor is a room temperatureelectrochemical gas sensor using a solid polymer proton conductor with aplanar sensor design. Response of this further improved sensor to CO wasvery weak, and was in the nA range even as a DC power source wasapplied. Apparently, the internal resistance of the sensor was toolarge. Calculations based on this further improved sensor dimensionsindicates that the ionic resistance of the proton conductor membrane isabout 400 K-ohm, which is too large to generate a usably strong signal.Further development and improvement of the planar CO gas sensor, whichincorporated a sensing mechanism, resulted in performance that was stillin nA range of sensor response.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of this invention to provide a low cost room temperatureelectrochemical gas sensor, for carbon monoxide and other toxic gases,having a low ionic resistance, a rapid response, and a strong signal tothe detection of gaseous CO in the ambient. The toxic gases that can besensed by the inventive sensor, each of which is referred to herein asan analyte gas, include H₂, H₂S, H₂O vapor, and NO_(x) concentrations.

The inventive electrochemical sensor has both a sensing electrode and acounter electrode. Each of the sensing and counter electrodes can bemade of mixed protonic-electronic conductors so as to encourage a highsurface area for reactions at the electrodes, which cause fast analytegas reaction kinetics and a continuity in the transport of electricalcharges so as to avoid polarization effects at the electrodes, thusachieving a fast and strong signal response by the sensor in thepresence of the analyte gas.

A further aspect of the inventive gas sensor is that only two electrodesare required, whereas prior art gas sensors require three electrodes anda DC power supply.

These objects have been achieved by using a novel button sensor design,which may include mixed proton-electron conductive electrodes, variousembodiments of which may also include an electrochemical analyte gaspump to transport analyte gas away from the counter electrode side ofthe gas sensor. While the inventive sensor is referred to herein as a COsensor, it is contemplated that the inventive sensor is also capable ofsensing other toxic analyte gases disclosed herein.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered bycounter-reference to specific embodiments thereof which are illustratedin the appended drawings. Understanding that these drawings depict onlya typical embodiment of the invention and are not therefore to beconsidered to be limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 is an electrochemical gas sensor showing the transport processesof both a potentiometric CO sensor and an amperometric CO sensor, wherehydrogen protons are conducted through a protonic conductive membranesituated between sensing and counter electrodes, where electrons travelbetween said electrodes away from the protonic conductive membrane,where the sensing electrode is the locus of the oxidation reaction ofcarbon monoxide instigated by a catalyst, and the counter electrode isthe locus of the synthesis of water from the products of theelectrochemical reaction of the sensor.

FIG. 2 shows an embodiment for the inventive electrochemical gas sensorthat is contained in a can having a cap which encloses a counterelectrode and a sensing electrode, where a protonic conductor issituated between the electrodes, which electrodes are separated byinsulated packing material within the can and cap container arrangement.FIG. 2 also sets forth optional electrical switching circuitry thatenables intermittent CO pumping away from the counter electrodealternating with direct sensing of CO through the switch mechanism.Without such switching circuitry, the inventive electrochemical COsensor depicted in FIG. 2 continuously senses CO.

The purpose of a CO electrochemical pump is to prevent an accumulationat the counter electrode of the inventive CO sensor. The CO pump lendsincreases stability to the sensor response in that the sensor responsehas less of a propensity to shift with time as in prior art CO sensingdevices. When DC power is used as the motivator for the electrochemicalCO pump, the passing of electrons from the sensing electrode to thereference electrode is enhanced. By reversing the DC power, the CO iskept away from the reference electrode and does not cause a buildup ofCO on the back side of the sensing electrode.

FIGS. 3A and 3B respectively depict sensor output where CO isintermittently pumped away from the counter electrode depicted in FIG. 2when the switching circuitry therein is activated, where a DC pulsepower is applied across the protonic conductive membrane to pump CO, andthe sensing response is recorded when the DC pulse power is not appliedacross the protonic conductive membrane. When the DC pulse power isapplied, permeated CO gas is catalytically converted to protons and thenis pumped out of the reference electrode side of the inventiveelectrochemical sensor depicted in FIG. 2.

FIG. 4 shows an alternative embodiment of the inventive electrochemicalsensor, further featuring a CO pump structure. The electrochemicalsensor depicted in FIG. 4 has four electrodes attached to a protonicconductive membrane, two of which are normal sensing and counterelectrodes, and the other two electrodes are used to pump permeated COout of the counter electrode side of the electrochemical cell. In thisalternative embodiment of the inventive CO sensor, DC power can beapplied in either a pulse mode or a constant mode. The electrochemicalsensor 15 is enclosed within an electrically insulated cap and candesign.

FIG. 5 shows a further embodiment of the inventive electrochemical COsensor, having two protonic conductive membranes, the first membranebeing used to sense CO, and the second membrane being used to pumppermeated CO out of the counter electrode side of the electrochemical COsensor.

FIG. 6A shows five sensor current signal responses in environmentalparameters of 22° C. and 20% relative humidity, as measured in currentversus CO concentration, where sensors having a larger size show ahigher current responses.

FIG. 6A also shows current signal response for varied concentrations ofmethane and propane, to demonstrate that sensor response is notinterfered with by increasing concentrations of methane and propane.

FIG. 6B shows sensor response with respect to time for varyingconcentrations of carbon monoxide in an environment of 19°-24° C., andrelative humidity from 23-29%. FIG. 6B shows voltage signal response ofa two membrane sensor similar to that depicted in FIG. 5.

FIG. 7 shows a graph of voltage signal response as a function of timefor an embodiment of the inventive electrochemical CO sensor in anenvironment of varying CO concentration.

FIG. 8 shows a plot of sensor voltage signal response versus COconcentration at dual relative humidities.

FIG. 9 shows an electrically conductive electrode in amplified view ofthe materials therein, having a current collector, electron conductivephases, gas phases, three-phase contact areas, and a protonic conductivemembrane.

FIG. 10 shows a mixed protonic-electronic conductive electrode inamplified view of the materials therein having a current collector,protonic and electronic conductive phases, gas phases, three-phasecontact areas, and a protonic conductive membrane.

FIG. 11 is an alternative embodiment of the inventive gas sensor havingthree electrodes which are a sensing electrode, a counter-electrode, anda counter electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventive CO sensor is a solid proton conductor room temperaturehaving a fast and high signal response. To achieve a fast detection timeand a high signal response, it is desirable to provide a CO sensorhaving a low bulk ionic resistance. Bulk ionic resistance R_(bulk) ofthe inventive sensor is equal toR_(bulk)=R_(o)d/s  (3)where R_(o) is the ionic resistivity of the protonic conductivemembrane, S is the cross section area of the protonic conductivemembrane between the two electrodes, and d is the thickness of theprotonic conductive membrane.

Resistance of an electrochemical cell includes at least threecomponents: 1) bulk ionic resistance of the membrane, 2) interfaceresistance between the membrane and electrodes, and 3) electronicresistance of the electrodes. The bulk ionic resistance of the sensor isreduced to about 1 ohm by the inventive button sensor design, such thatR bulk is not a performance limit. Electronic resistivity of theelectrodes is in order of 10-5 ohm.cm and obviously is not a performancelimit. Therefore, the interface resistance, which is relative to theavailable three-phase contact area, becomes the performance limit.

Assuming that a button NAFION™ CO sensor is exposed to 1,000 ppm CO withair. The Nernst Potential of the sensor is about 200 mV according to ourexperiment data. If the interface resistance is insignificant, theresponse shorting current would be about 200 mA (or 250 mA/cm²). For thereal case, we only recorded a response current less than 1 mA/cm²

due to existing a large interface resistance. The interface resistanceof the sensor according to this invention has been reduced byintroducing our mixed proton-electronic conductor.

Two alternative embodiments of the inventive CO sensor are depicted inFIG. 2 as button sensor 10. Button sensor 10 has a protonic conductivemembrane 12 situated between counter electrode 14 and sensing electrode16. Button sensor 10 has a “button” shape due to a metal can having anopening which is covered by a metallic cap 32. Can 30 has air samplingholes 38 therein so as to provide a venting of sensing electrode 16 tothe ambient. An insulation packing material 34 electrically insulatescap 32 from can 30. A washer 36 separates protonic conductive membrane12 from can 30. A voltage meter 44 measures potential differencesbetween electrical leads 20, 22 in a potentiometric CO gas sensorembodiment.

In a second embodiment shown in FIG. 2, electrical leads 20, 22 areconnected to a switching mechanism for button sensor 10 made up of aswitch 40 that is opened and closed by unit 42 so as to alternativelyprovide a power source 44 in electrical communication with cap 32 andcan 30 of button sensor 10. The purpose of the foregoing electricalswitching circuitry is to provide a switchable CO pump to button sensor10 so as to direct CO away from counter electrode 14 before and aftersensing and measuring CO concentration with button sensor 10.Alternative, as shown in connection with FIG. 1, an amp meter 24, incombination with a resistor R_(L) 26 provides an amperometric CO sensorembodiment when button sensor 10 of FIG. 2 incorporates the circuitryseen in FIG. 1. If switchable CO pump circuitry is not included in theembodiment of button sensor 10 shown in FIG. 2, then continuous sensingwithout CO pumping is performed by button sensor 10.

The amperometric sensor also can be combined with an electrochemical COpump, as defined hereinafter, and accurate response will be achieved insuch combined sensors. In the inventive button sensor design as shown inFIG. 2, it is desirable that both area and thickness parameters areoptimized. It is beneficial for button CO sensor 10 to have a 0.1 mm-1mm thick NAFION™ protonic membrane, and that the diameter of sensing andcounter electrodes 16, 15 be approximately 1 mm to 15 mm. Preferably,button CO sensor 10 has a 0.17 mm thick NAFION™ protonic membrane or thelike with 10 mm diameter sensing and counter electrodes 16, 14, whichresults in a bulk ionic resistance of 1.0 ohm. The proton conductor forboth the sensing and counter electrodes is preferably a copolymer basedon a tetrafluorethylene backbone with a side chain of perfluorinatedmonomers containing sulfonic or carboxyic acid groups, especially aNAFION™ 117 material from Du Pont, a R4010 or a R1010 material form PallRAI Manufacture Co., or the like.

Protonic conductors membranes are usually slightly permeable to CO gas.When a membrane is under a carbon monoxide partial pressure difference,a very small amount of carbon monoxide will permeate across the membraneinto the counter electrode side.

Influence of the CO permeation to sensor response usually isinsignificant because this very small amount of permeated CO isinstantly converted into carbon dioxide at the reference electrode. If aprecision CO concentration detection is needed, CO concentration in thecounter electrode can be minimized by attaching an electrochemical COpump to the sensor according to this invention. The purpose of anelectrochemical pumping circuitry is to prevent the buildup of CO gas atthe counter electrode side of the sensor so that a precision COdetection is achieved.

Protonic conductive membrane 12 may be substantially composed of asolid, perfluorinated ion-exchange polymer, or a metal oxide protonicconductor electrolyte material. The following table serves as a furtherexample of solid state protonic conductor which can be used at roomtemperature in the inventive gas sensor.

MATERIALS 1. H₃Mo₁₂PO₄₀•29H₂O 6. NAFION ™ DuPont.(US) 2. H₃W₁₂PO₄•29H₂O7. C membrane Chlorine Engineer's (Japan) 3. HUO₂PO₄•4H₂O 8. XUS-1304.10Dow (US) 4. Zr(HPO₄)₂•3H₂O 9. R4010-55 PALL RAI Manufacturing Co. (US)5. Sb₂O₅•4H₂OProtonic conductive membrane 12 is preferably constructed of materials6, 7, 8, or 9 which are unreinforced film of perfluorinated copolymers.

FIGS. 3A and 3B show the alterative operation of an electrochemical COpump and CO sensor response, the circuitry for which is depicted in FIG.2. A DC pulse power is applied across protonic conductive membrane 12seen in FIG. 2. This DC pulse power pump voltage is seen in FIG. 3A atcounter line 62 showing a 2 V voltage, and the sensor response is seenby counter line 68 in FIG. 3B. FIG. 3B indicates that the sensor ofoutput is off during the DC pulse power application to protonicconductive membrane 12. During the electrochemical CO pump process,permeated CO gas is catalytically converted to protons, which are thenpumped out of the side of button sensor 10 associated with referenceelectrode 14. counter line 64 in FIG. 3A shows no pump voltage appliedto membrane 12, and counter line 70 in FIG. 3B shows the sensor outputreading of button sensor 10 when the electrochemical CO pump is notactive.

As seen in FIG. 3A, a pump voltage of 2 V is preferred for theelectrochemical CO pump stage. The pump voltage applied in FIG. 3A andthe sensor output reading voltage seen in FIG. 3B are representative ofthe intermittent sensor output and electrochemical CO pumpingoperational capabilities of button sensor 10 seen in FIG. 2.

FIG. 4 features counter numerals similar to FIG. 2, with identicalcounter numerals referring to similar structures performing similarfunctions. FIG. 4 shows an alternative embodiment of button sensor 10,which lacks the DC power switching circuity shown in FIG. 2, butincludes pumping electrodes 15, 17 interfacing protonic conductivemembrane 12. The purpose of pumping electrodes 15, 17 is to continuouslypump CO away from counter electrode 14 side of button sensor 10 whilecontinuously sensing the presence of CO gas in the ambient. Thiscontinuous pumping of CO away from the side of button sensor 10 wherecounter electrode 14 is located serves to give stability to the sensorsignal response to CO concentration in the ambient. The DC power sourcecan be operated in either “pulse” mode to pump CO, or in the “on” modeto sense CO concentration. In button sensor 10, depicted in FIG. 4, bothcan 30 and cap 32 are preferably made of electrically insulativematerials.

A further embodiment of the inventive CO sensor is seen in FIG. 5 as abutton sensor 110. Button sensor 110 has two protonic conductivemembranes that prevent interference with button sensor 110's response tothe detection of CO concentrations. Button sensor 110 features ametallic shell 130 having a porous centerpiece 135 terminated at eitherend thereof by walls having a concave configuration. A bottom cap 132Aand a top cap 132B are both porous, respectively having holes 138A, 138Btherethrough, and being comprised of a metallic substance, preferablysimilar to can 130. Insulation material 134 insulates a first protonicconductive membrane 122 and a second protonic conductive membrane 112.Washer 136 separates porous centerpiece 135 from first and secondprotonic conductive membranes 122, 112. A sensor electrode 116 is on anopposite side of first protonic conductive membrane 122 from a counterelectrode 114. Counter electrode 114 contacts with metallic centerpiece135 of can 130. Pumping electrodes 115, 117 are in contact with oppositesides of second protonic conductive membranes 112.

A DC power source 140 is in electrical contact with pumping electrode115 and metallic can 130 through electrical contacts 146 and 144.Sensing electrode 116 is in contact with an electrical measurement means142 through electrical leads 148, 144. DC power supply 140 serves as aCO pump to button sensor 110. Electrical sensing means 142 is used tomeasure the response of button sensor 110 to concentrations of CO.

Sensing electrode 116 is exposed to the ambient through holes 138A.

CO pumping electrode 115 is exposed through holes 138B to a sealedchamber 115A which serves as a counter environment.

Sensing electrode 116 is exposed to the ambient through holes 138A.First protonic conductive membrane 122 performs the function, incombination with counter and sensing electrodes 114, 116, of sensing COconcentration through the conduction therethrough of protons. Secondprotonic conductive membrane 112, in combination with pumping electrodes115, 117, performs the function of pumping CO out of the side of buttonsensor 110 associated with counter electrode 114 so as to stabilize thesensor response of button sensor 110 upon the detection of aconcentration of CO in the ambient.

FIG. 6A shows the results of five different embodiments of the inventiveCO sensor. As can be seen by FIG. 6A, current responds linearly in alogarithmic scale to ambient concentration of CO. FIG. 6A also showsthat increasing concentrations of methane and propane do not interferewith CO sensing of the inventive sensor.

FIG. 6B shows sensor voltage response with respect to time of theinventive two protonic conductive membrane gas sensor seen in FIG. 5.Reference point 90 shows zero time with a CO concentration of 1-2 ppm.Reference point 92 shows an environment of 5-10 ppm CO after a period of1,000 minutes. At reference point 92 on FIG. 6B, an injection of 100 ppmCO is made into the environment such that sensor response maximizes atreference point 94 on FIG. 6B. At reference point 94 on FIG. 6B, theatmosphere is seen to be opened up to clean air and the sensor responsedecreases to reference point 94A on FIG. 9B after a period of 3 minutes.At reference point 95 on FIG. 6B, background CO concentration rises from15-20 ppm until sensor response maximizes at reference point 96 on FIG.6B. Clean background air is introduced into the environment at referencepoint 96 on FIG. 6B such that sensor response declines to referencepoint 98 on FIG. 6B. FIG. 6B reflects environmental parameters of19°-24° C. and 23-29% relative humidity. Such a sensor voltage responseis seen in a nonlogrithmic scale in FIG. 6B.

FIG. 7 shows the voltage response of the inventive sensor to changes inthe atmospheric concentration of CO, where a response of 120 mV is seenat counter point 72 in a concentration of 500 ppm CO, which responserequired 10 seconds to achieve from reference point 70 where aninsignificant concentration of CO was present in the ambient. Thesensing electrode was exposed to an environment containing CO, whereasthe counter electrode side was exposed to clean air. As can be seen fromFIG. 7, the inventive CO sensor has a rapid response time in comparisonto prior art CO sensors.

FIG. 8 shows the characteristic of the inventive CO sensor with respectto its independence of varying relative humidity environments. In apotentiometric embodiment of the inventive CO gas sensor, as can be seenfrom FIG. 8, relative humidity does not interfere with the linear natureof the sensor response in increasing environments of CO concentration.

The ability of the inventive CO sensor to avoid interference withrelative humidity is that, with increased relative humidity, bulk ionicresistance of the inventive CO sensor goes down as current flowincreases. The resistance decrease and current increase areproportionally the same. Thus, voltage, or sensor response, remainsconstant as evidenced by the equation V=RI.

In the inventive CO sensor, the sensing electrode is exposed to anenvironment containing CO, whereas the counter electrode side is sealedair-tight. The sensing mechanism of this sensor is essentially the sameas that of the sensor with an opened reference electrode. The protonicconductive membrane can be as thin as 0.2 mm so that the reactant oxygenand the produced water permeate the membrane. A small part of CO gasalso permeates through the membrane, but the permeated CO is consumed bythe reaction with oxygen electrochemically and catalytically at thecounter electrode.

FIG. 9 is an amplified view of an electrically conductive electrodehaving protonic conductive membrane 12, a current collector electricallead 22, and an electron conductive phase material 82 therebetween.Electron conductive phase material 82 has a plurality of gaps 80interstitially placed between particles of electron conductive phasematerial 82. A plurality of three-phase contact areas 86 exists andinterfaces between protonic conductive membrane 12 and electronconductive phase material 82. CO gas in the ambient coming in contactwith electron conductive phase material 82 produces electrons which aredrawn to current collector electrical lead 22. CO gas in the ambientcoming in contact with the interface of electron conductive phasematerial 82 and protonic conductive membrane 12 at three-phaseconductive contact area 86 will produce hydrogen ions, or protons, whichare conducted through protonic conductive membrane 12. As can be seenfrom FIG. 9, the creation of hydrogen ions occurs only at the surface ofprotonic conductive membrane 12 at three-phase contact area 86. Thus,there is limited surface at which the creation of hydrogen ions can takeplace in the embodiment of the electronically conducted electrode shownin FIG. 9.

FIG. 10 shows a mixed protonic-electronic conductive electrode having aprotonic conductive membrane 12, a current collector electrical lead 22,and a variety of amplified particles therebetween and consisting of anelectronic conductive phase material 82, and a protonic conductive phasematerial 84. Between particles of protonic conductive phase material 84and electronic conductive phase material 82, there are gaps 80 whichrepresent the pores between the particles situated between currentcollector electrical lead 22 and protonic conductive membrane 12.Electrons are transmitted to current collector electrical lead 22 whenCO gas in the ambient comes in contact with three-phase contact area 86.Hydrogen ions are transported to protonic conductor membrane 12 when COgas in the ambient comes in contact with three-phase contact area 86.The creation of both hydrogen ions and electrons occurs at each of theplurality of three-phase contact areas 86 shown in FIG. 10. Neitherelectrons nor hydrogen ions are created at interface 88 which issituated between protonic conductive membrane 12 and protonic conductivephase material 84. Similarly, no reaction to create electrons orhydrogen ions occurs at an interface 88 between current collectorelectric lead 22 and electronic conductor phase material 82.

As can be seen from FIG. 10, the creation of hydrogen ions occurs in thethree-dimensional area between current collector electrical lead 22 andprotonic conductive membrane 12. Thus, the surface area available tocreate hydrogen ions is greater in the electrodes seen in FIG. 10 ascompared to the electrode seen in FIG. 9. This additional surface areafor creation of hydrogen ions is due to the presence of protonicconductive phase material 84 in the electrode above protonic conductivemembrane 12. Conversely, FIG. 9 does not contain any protonic conductivephase material situated on and above protonic conductive membrane 12.

The mixed conductor material found in the electrode seen in FIG. 10, byproviding a high surface area for the CO oxidation reaction in thesensing electrode side, produces a faster and more sensitive sensorresponse than the electrode seen in FIG. 9.

The mixed conductor material found in the electrode seen in FIG. 10, byproviding a high surface area for the H₂O formation reaction in counterelectrode side, produces a faster and more sensitive sensor responsethan the electrode seen in FIG. 9.

FIG. 11 depicts an embodiment of the inventive sensor having threeelectrodes. While the foregoing embodiments of the inventive sensor usedonly two electrodes, and thereby resulted in cost savings, athree-electrode embodiment of the invention is seen as button sensor 10in FIG. 11. Counter numerals in FIG. 11 identical to counter numerals inFIG. 2, represent similar structures performing similar functions.

Button sensor 10 in FIG. 11 has a counter electrode 14, and a referenceelectrode 15 between an electrical insulation cap 32 and a protonicconductive membrane 12. On an opposite side of protonic conductivemembrane 12 is a sensing electrode 16. Sensing electrode 16 is vented tothe ambient through holes 38 on a bottom side of can 30. Gasket 36 andinsulation material 34 keep counter electrode 14 and reference electrode15 air tightly sealed.

Electrical lead 20A electrically contacts sensing electrode 16 throughcan 30. Electrical lead 20A is connected to an amp meter 24 which is inseries with a DC power source 42. DC power source 42 is connected toamplifier 45, which amplifier 45 is connected through to electrical lead20B, which penetrates cap 32 into counter electrode 14. Amplifier 45 iselectrically connected to an electrical lead 20C which penetratesthrough can 30 into counter electrode 14. The function of the electricalcircuitry shown in FIG. 11 is to set the electrical potential of sensingelectrode 16 to a given constant value with respect to referenceelectrode 15. Although potential differences exist between sensingelectrode 16 and reference electrode 15, there is no current flowingtherebetween. At the same time, the electrical current passing betweensensing electrode 16 and counter electrode 14, which is indicative of COconcentration, is recorded by amp meter 24.

The inventive CO gas sensor using the mixed protonic-electronicconductive materials in the electrodes with high surface area of 100 to1000 M²/g shows a shorting current as high as 150 μA/cm² to 1,000 ppmCO, which is at least two orders of magnitude higher compared to thesensors with electronic conductive electrodes according to prior art. Apreferred composition of such electrodes is as follows:

COUNTER ELECTRODE SENSING ELECTRODE 7.5 wt % Ru oxide 20 wt % Pt-black67.5 wt % carbon 55 wt % carbon 25 wt % NAFION ™ 25 wt % NAFION ™

Other compositions of such electrodes are as follows:

COUNTER ELECTRODE SENSING ELECTRODE Pd 20 wt % Pd 20 wt % Carbon 60 wt %Carbon 60 wt % Sb₂O₅•4H₂O 20 wt % Sb₂O₅•4H₂O 20 wt % Rb 25 wt % Pd 25 wt% Carbon 50 wt % Ni 50 wt % R4010-55 25 wt % R4010-55 25 wt % 10 wt % Pton vulcan carbon 10 wt % Pt on vulcan carbon XC72 25 wt % XC72 25 wt %NAFION ™ 25 wt % NAFION ™ 25 wt % Ti 50 wt % Ni 50 wt % 20 wt % Pt-Black20 wt % Pt-black 55 wt % carbon 55 wt % carbon 25 wt % NAFION ™ 25 wt %NAFION ™

The role of platinum in the sensing electrode is to favor the COdecomposition reaction (1) whereas Ru oxide in the counter electrode isto favor the water formation reaction (2). According to this invention,the Ru oxide, instead of expensive platinum and the like, as reported inprior art, shows excellent CO sensing performance.

It is also contemplated that the electrodes disclosed herein can becomposed substantially of carbon, noble metals, or conductive metaloxides. The electrical conducting material in electrodes disclosed hereis preferably a proton-electron mixed conductive material having 10-50wt% of a proton conductor material and 50-90 wt% of a first and a secondelectrical conductor material. The proton conductor material for theelectrodes disclosed herein is preferably a copolymer having atetrafluorethylene backbone with a side chain of perfluorinated monomerscontaining at least one of a sulfonic acid group or carboxylic acidgroup. Preferably, one of the first and second electrical conductormaterials for the sensing electrodes disclosed herein is 50-99 wt% ofcarbon black, and the other of the first and second electrical conductormaterials for the sensing electrodes disclosed herein is 1-50 wt% ofplatinum. Also preferably, one of the first and second electricalconductor materials for the counter electrode is 50-99 wt% of carbonblack, and the other of the first and second electrical conductormaterials for the counter electrode is 1-50 wt% of Ru oxide.

In a composition of 25 wt% protonic conductor in electrodes, which is aphysically continuous phase, there is proton conduction, whereas therest of the phases in electrodes provide electronic conduction as wellas catalytic activity. If without 25 wt% proton conductor in electrodes,the electrodes were only an electronic conductor, and the reactions (1)and (2), above, would only occur at three-phase contact area 86 seen inFIG. 9, which is a very limited small area When the electrodes are madeof mixed conductors according to this invention, the reactions (1) and(2) will occur on all surface of the electrodes. Therefore, by usinghigh surface area mixed conductive electrodes (100 to 1,000 M²/g) seenin FIG. 10, fast CO reaction kinetics at the interface are achieved andstrong signal response is obtained.

While the inventive gas sensor can be used to measure CO concentration,it is also capable of measuring other gases such as H₂, H₂S, H₂O vapor,and NO_(x) concentrations.

Various protonic conductors, including organic protonic conductors andinorganic protonic conductors, can be used in the sensor according tothis invention. In what follows, a copolymer protonic conductivemembrane based on a tetrafluoroethylene backbone with a side chain ofperfluorinated monomers containing sulfonic acid group is used herein asan example of the fabrication of the inventive sensor.

To prevent deterioration of the polymer membrane in the subsequentwetting/drying steps, the membrane must be first converted from theproton form to the sodium form by the following steps A:

-   A. The polymer membrane is soaked in lightly boiling dilute NaOH    solution for 1-3 hours. It is then rinsed first in tap water for    0.5-3 hours, then in deionized water for 10-30 minutes, and is then    laid out on a rack to air dry.-   B. The materials for the preferred mixed conduction electrodes are    as follows: Pt/carbon powder, carbon powder, Ru oxide powder,    solubilized polymer solution, Glycerol, NaOH solution, and deionized    water.-   C. The steps for fabrication are as follows:    -   1. Pre-mix deionized water and glycerol in 20-30% weight ratio,        and store the mixture in a container;    -   2. Weigh an appropriate amount of Pt/carbon powder into a clean        container;    -   3. Weigh an appropriate amount of 5%wt polymer solution, and add        to material in step C.2, and then mix. Typically, add 1-3 parts        5%wt NAFION™ solution (on a dry polymer basis) to 3-5 parts        Pt/carbon powder;    -   4. Weigh and add an appropriate amount of water/glycerol mixture        to mixture in step C.3, and then mix. Typically, add 25-35 parts        water/glycerol mixture to one pan Pt/carbon powder;    -   5. Weigh and add an appropriate amount of 1-2 Moles NaOH to the        mixture in step C.4, and then mix. Typically, add 1-2 parts 1-2        Moles NaOH to 9-15 parts 5% wt polymer solution; and further mix        the wet electrode mixture ultrasonically for 60 minutes.-   D. For Carbon/Ru Oxide electrode preparation, the following steps    are taken:    -   1. Pre-mix the deionized water and glycerol in 20-30% weight        ratio, store the mixture in a container, and set aside;    -   2. Weigh an appropriate amount of carbon powder and Ru oxide        into a clean container;    -   3. Weigh an appropriate amount of 5%wt polymer solution, and add        to the material in step D.2, and then mix. Typically, add 1-3        parts 5%wt polymer solution (on a dry polymer basis) to 3-5        parts carbon/Ru oxide powder;    -   4. Weigh and add an appropriate amount of water/glycerol mixture        to mixture in step D.3, and then mix. Typically, add 25-35 parts        water/glycerol mixture to 1 part carbon/Ru oxide powder;    -   5. Weigh and add an appropriate amount of 1-2 Moles NaOH to the        mixture in step C.4, and then mix. Typically, add 1 part 1-2        Moles NaOH to 9-15 parts 5%wt polymer solution; and further mix        the wet electrode mixture ultrasonically for 60 minutes.-   E. For Pt/Carbon Electrode application drying, the following steps    are taken:    -   1. Re-mix the wet electrode mixture ultrasonically for at least        30 minutes prior to use;    -   2. Fill the dispensing machine tubing with the Pt/carbon wet        electrode mixture;    -   3. Dispense the wet electrode mixture to the surface of the        membrane at the desired location; and    -   4. Place the membrane/electrode in an oven at 100°-170° C. for        10-60 minutes.-   F. For Carbon/Ru Oxide Electrode application drying, the following    steps are taken:    -   Repeat step A on the opposite side of the membrane.-   G. For acidification, the following steps are taken:    -   1. For Ion-Exchange, soak membrane/electrodes in lightly boiling        dilute MH2SO4 solution for 1-3 hours.    -   2. For cleaning, rinse the membrane/electrodes in deionized        water;    -   3. For drying, dry the membrane/electrodes in air, or air dry        then desiccate overnight, or place in a 30°-50° C. oven for 1-3        hours before cutting to the final dimensions.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. An electrochemical gas sensor for quantitative measurementof a gas in an ambient atmosphere comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; a first protonicconductive electrolyte membrane in between and in contact with thesensing and counter electrodes, and having a thickness in the range ofapproximately 0.1 mm to 1 mm; the sensing electrode reacting with thegas to produce a change in an electrical characteristic between thesensing electrode and the counter electrode; means for electricalmeasurement; said sensing and counter electrodes each having a diameterin the range of approximately 1 mm to 15 mm, and being electricallyconnected to said electrical measurement means; whereby, in a positiveambient concentration of said gas, said electrical measurement meansdetects changes in said electrical characteristic.
 2. Theelectrochemical gas sensor as defined in claim 1, An electrochemical gassensor for quantitative measurement of a gas in an ambient atmospherecomprising: a porous mixed ionic-electronic conductive sensing electrodehaving both an electronic conducting material and an ionic conductingmaterial; a porous mixed ionic-electronic conductive counter electrodehaving both an electronic conducting material and an ionic conductingmaterial; wherein the electronic and ionic conducting materials of saidsensing and counter electrodes are proton-electron mixed conductivematerials having 10-50 wt % of a proton conductor material and 50-90 wt% of a first and a second electrical conductor materials; a firstprotonic conductive electrolyte membrane in between and in contact withthe sensing and counter electrodes, and having a thickness in the rangeof approximately 0.1 mm to 1 mm, the sensing electrode and the counterelectrode being on opposite sides of the first protonic conductiveelectrolyte membrane; the sensing electrode reacting with the gas toproduce a change in an electrical characteristic between the sensingelectrode and the counter electrode; means for electrical measurement;said sensing and counter electrodes each having a diameter in the rangeof approximately 1 mm to 15 mm, and being electrically connected to saidelectrical measurement means; whereby, in a positive ambientconcentration of said gas, said electrical measurement means detectschanges in said electrical characteristic; the electrochemical gassensor further comprising: means for applying DC power across theprotonic conductive electrolyte membrane; an electrical connectionbetween the sensing electrode, the counter electrode, and the means forapplying DC power across the protonic conductive electrolyte membrane;and switch means for alternating an electrical connection between thesensing electrode and counter electrode from the electrical measurementmeans to the means for applying DC power across the protonic conductiveelectrolyte membrane; whereby the gas is transported away from thecounter electrode when the means for applying DC power across theprotonic conductive electrolyte membrane applies a DC power to thesensing and counter electrodes.
 3. The electrochemical gas sensor asdefined in claim 1, wherein said sensing and counter electrodes comprisecarbon.
 4. The electrochemical gas sensor as defined in claim 1, whereinsaid sensing and counter electrodes comprise noble metals.
 5. Theelectrochemical gas sensor as defined in claim 1, wherein said sensingand counter electrodes comprise conductive metal oxides.
 6. Theelectrochemical gas sensor as defined in claim 1, wherein the protonicconductive electrolyte membrane is substantially comprised of a solid,perfluorinated, ion-exchange polymer.
 7. The electrochemical gas sensoras defined in claim 1, wherein the protonic conductive electrolytemembrane is a metal oxide protonic conductor electrolyte membrane. 8.The electrochemical gas sensor as defined in claim 1, wherein theelectrochemical gas sensor is adapted to detect CO.
 9. Theelectrochemical gas sensor as defined in claim 1, wherein theelectrochemical gas sensor is adapted to detect NO_(x).
 10. Theelectrochemical gas sensor as defined in claim 1, wherein theelectrochemical gas sensor is adapted to detect hydrogen.
 11. Theelectrochemical gas sensor as defined in claim 1, wherein theelectrochemical gas sensor is adapted to detect H₂S.
 12. Theelectrochemical gas sensor as defined in claim 1, wherein theelectrochemical gas sensor is adapted to detect H₂O vapor.
 13. Theelectrochemical gas sensor as defined in claim 1, wherein the sensingand counter electrodes have a diameter of about 10 mm, and the protonicconductive electrolyte membrane has a thickness of about 0.17 mm. 14.The electrochemical gas sensor as defined in claim 1, wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are a proton-electron mixed conductive material having 10-50wt% of a proton conductor material and 50-90 wt% of a first and a secondelectrical conductor materials.
 15. The electrochemical gas sensor asdefined in claim 14, wherein the proton conductor material for both thesensing and counter electrodes is a copolymer having atetrafluorethylene backbone with a side chain of perfluorinated monomerscontaining at least one of a sulfonic acid group or a carboxylic acidgroup.
 16. The electrochemical gas sensor as defined in claim 14,wherein one of the first and second electrical conductor materials forthe sensing electrode is 50-99 wt% of carbon black, and the other of thefirst and second electrical conductor materials for the sensingelectrode is 1-50 wt% of platinum.
 17. The electrochemical gas sensor asdefined in claim 14, wherein one of the first and second electricalconductor materials for the counter electrode is 50-99 wt% of carbonblack, and the other of the first and second electrical conductormaterials for the counter electrode is 1-50 wt% of Ru oxide.
 18. Theelectrochemical gas sensor as defined in claim 1, An electrochemical gassensor for quantitative measurement of a gas in an ambient atmospherecomprising: a porous mixed ionic-electronic conductive sensing electrodehaving both an electronic conducting material and an ionic conductingmaterial; a porous mixed ionic-electronic conductive counter electrodehaving both an electronic conducting material and an ionic conductingmaterial; wherein the electronic and ionic conducting materials of saidsensing and counter electrodes are proton-electron mixed conductivematerials having 10-50 wt % of a proton conductor material and 50-90 wt% of a first and a second electrical conductor materials; a firstprotonic conductive electrolyte membrane in between and in contact withthe sensing and counter electrodes, and having a thickness in the rangeof approximately 0.1 mm to 1 mm, the sensing electrode and the counterelectrode being on opposite sides of the first protonic conductiveelectrolyte membrane; the sensing electrode reacting with the gas toproduce a change in an electrical characteristic between the sensingelectrode and the counter electrode; means for electrical measurement;said sensing and counter electrodes each having a diameter in the rangeof approximately 1 mm to 15 mm, and being electrically connected to saidelectrical measurement means; whereby, in a positive ambientconcentration of said gas, said electrical measurement means detectschanges in said electrical characteristic; wherein the electrochemicalgas sensor further comprises: first and second porous mixedionic-electronic conductive pump electrodes each having both anelectronic conductive material and an ionic conductive material, each ofsaid first and second pump electrodes being separate from said sensingand counter electrodes and situated on opposite sides of and in contactwith said protonic conductive electrolyte membrane; means for applying aDC power across the membrane; said first and second pump electrodeshaving in electrical connection therebetween said means for applying DCpower across the membrane; whereby the gas is transported away from thecounter electrode when said means for applying DC power across themembrane applies a DC power to the first and second pump electrodes. 19.The electrochemical gas sensor of claim 18, wherein the electronic andionic conducting materials of the first and second pumping electrodescomprise carbon.
 20. The electrochemical gas sensor as defined in claim18, wherein the electronic and ionic conducting materials of the firstand second pumping electrodes comprise noble metals.
 21. Theelectrochemical gas sensor as defined in claim 18, wherein theelectronic and ionic conducting materials of the first and secondpumping electrodes comprise conductive metal oxides.
 22. Theelectrochemical gas sensor as defined in claim 18, wherein the first andsecond pumping electrodes have a diameter of about 10 mm, and the firstprotonic conductive electrolyte membrane has a thickness of about 0.17mm.
 23. The electrochemical gas sensor as defined in claim 18, whereinthe electronic and ionic conducting materials of said first and secondpumping electrodes are a proton-electron mixed conductive materialhaving 10-50 wt% of a proton conductor material and 50-90 wt% of a firstand a second electrical conductor materials.
 24. The electrochemical gassensor as defined in claim 23, wherein the proton conductor material forboth the first and second pumping electrodes is a copolymer having atetrafluorethylene backbone with a side chain of perfluorinated monomerscontaining at least one of a sulfonic acid group or a carboxylic acidgroup.
 25. The electrochemical gas sensor as defined in claim 23,wherein one of the first and second electrical conductor materials forthe first pumping electrode is 50-99 wt% of carbon black, and the otherof the first and second electrical conductor materials for the firstpumping electrode is 10 to 50 wt% of platinum.
 26. The electrochemicalgas sensor as defined in claim 23, wherein one of the first and secondelectrical conductor materials for the second pumping electrode is 50-99wt% of carbon black, and the other of the first and second electricalconductor materials for the second pumping electrode is 10 to 50 wt% ofRu oxide.
 27. The electrochemical gas sensor as defined in claim 1, Anelectrochemical gas sensor for quantitative measurement of a gas in anambient atmosphere comprising: a porous mixed ionic-electronicconductive sensing electrode having both an electronic conductingmaterial and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm, the sensing electrode and the counter electrode being onopposite sides of the first protonic conductive electrolyte membrane;the sensing electrode reacting with the gas to produce a change in anelectrical characteristic between the sensing electrode and the counterelectrode; means for electrical measurement; said sensing and counterelectrodes each having a diameter in the range of approximately 1 mm to15 mm, and being electrically connected to said electrical measurementmeans; whereby, in a positive ambient concentration of said gas, saidelectrical measurement means detects changes in said electricalcharacteristic; wherein the electrochemical gas sensor furthercomprises: a second protonic conductive electrolyte membrane; first andsecond porous mixed ionic-electronic conductive pump electrodes eachhaving both an electronic conductive material and an ionic conductingmaterial, each of said first and second pump electrodes being separatefrom said sensing and counter electrodes and situated on opposite sidesof and in contact with said second protonic conductive electrolytemembrane; means for applying a DC power across said second protonicelectrolyte membrane; said first and second pump electrodes having inelectrical connection therebetween said means for applying DC poweracross said second protonic electrolyte membrane; whereby the gas istransported away from the counter electrode when said means for applyingDC power across said second protonic electrolyte membrane applies a DCpower to the first and second pump electrodes.
 28. The electrochemicalgas sensor as defined in claim 27, wherein the second protonicconductive electrolyte membrane is substantially comprised of a solid,perfluorinated, ion-exchange polymer.
 29. The electrochemical gas sensoras defined in claim 27, wherein the second protonic conductiveelectrolyte membrane is a metal oxide protonic conductor electrolytemembrane.
 30. An electrochemical gas sensor for quantitative measurementof a gas in an ambient atmosphere comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; a protonicconductive electrolyte membrane in between and in contact with thesensing and counter electrodes; the sensing electrode reacting beingcapable of detecting with the gas to produce a change in an electricalcharacteristic between the sensing electrode and the counter electrode;means for electrical measurement; said sensing and counter electrodeshaving electrically connected therebetween said means for electricalmeasurement; means for applying a DC pulse power source across themembrane; said sensing and counter electrodes having in electricalconnection therebetween said means for applying DC pulse power acrossthe membrane; and switch means for alternating the connection betweenthe sensing and counter electrodes from the electrical measurement meansto the means for applying a DC pulse power source across the membrane;whereby, in a positive ambient concentration of said gas, saidelectrical measurement means detects is capable of detecting changes insaid electrical characteristic when said switch means connects saidelectrical measurement means to the sensing and counter electrodes; andwhereby said means for applying a DC pulse power source across themembrane moves CO away from a side of the gas sensor where the counterelectrode is placed when said switch means connects said means forapplying a DC pulse power source across the membrane to the sensing andcounter electrodes.
 31. The electrochemical gas sensor as defined inclaim 30, wherein said sensing and counter electrodes comprise carbon.32. The electrochemical gas sensor as defined in claim 30, wherein saidsensing and counter electrodes comprise noble metals.
 33. Theelectrochemical gas sensor as defined in claim 30, wherein said sensingand counter electrodes comprise conductive metal oxides.
 34. Theelectrochemical gas sensor as defined in claim 30, wherein the protonicconductive electrolyte membrane is substantially comprised of a solid,perfluorinated, ion-exchange polymer.
 35. The electrochemical gas sensoras defined in claim 30, wherein the protonic conductive electrolytemembrane is a metal oxide protonic conductor electrolyte membrane. 36.The electrochemical gas sensor as defined in claim 30, wherein theelectrochemical gas sensor is adapted to detect CO.
 37. Theelectrochemical gas sensor as defined in claim 30, wherein theelectrochemical gas sensor is adapted to detect hydrogen.
 38. Theelectrochemical gas sensor as defined in claim 30, wherein theelectrochemical gas sensor is adapted to detect H₂S.
 39. Theelectrochemical gas sensor as defined in claim 30, wherein theelectrochemical gas sensor is adapted to detect H₂O vapor.
 40. Theelectrochemical gas sensor as defined in claim 30, wherein theelectrochemical gas sensor is adapted to detect NO_(x).
 41. Theelectrochemical gas sensor as defined in claim 30, wherein the sensingand counter electrodes have a diameter in a range of 1 mm to 15 mm, andthe protonic conductive electrolyte membrane has a thickness in a rangeof 0.1 mm-1 mm.
 42. The electrochemical gas sensor as defined in claim41, wherein the sensing and counter electrodes have a diameter of about10 mm, and the protonic conductive electrolyte membrane has a thicknessof about 0.17 mm.
 43. The electrochemical gas sensor as defined in claim30, wherein the electronic and ionic conducting materials of saidsensing and counter electrodes are a proton-electron mixed conductivematerial having 10-50 wt% of a proton conductor material and 50-90 wt%of a first and a second electrical conductor materials.
 44. Theelectrochemical gas sensor as defined in claim 43, wherein the protonconductor material for both the sensing and counter electrodes is acopolymer having a tetrafluorethylene backbone with a side chain ofperfluorinated monomers containing at least one of a sulfonic acid groupor a carboxylic acid group.
 45. The electrochemical gas sensor asdefined in claim 43, wherein one of the first and second electricalconductor materials for the sensing electrode is 50-99 wt% of carbonblack, and the other of the first and second electrical conductormaterials for the sensing electrode is 1-50 wt% of platinum.
 46. Theelectrochemical gas sensor as defined in claim 43, wherein one of thefirst and second electrical conductor materials for the counterelectrode is 50-99 wt% of carbon black, and the other of the first andsecond electrical conductor materials for the counter electrode is 1-50wt% of Ru oxide.
 47. An electrochemical gas sensor for quantitativemeasurement of a gas in an ambient atmosphere comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material and being exposedto the ambient atmosphere; a porous mixed ionic-conductive counterelectrode having both an electronic conducting material and an ionicconducting material; a first protonic conductive electrolyte membrane inbetween and in contact with the sensing and counter electrodes; thesensing electrode being capable of reacting with the gas to produce achange in an electrical characteristic between the sensing electrode andthe counter electrode; a second protonic conductive electrolytemembrane; first and second porous mixed ionic-electronic conductive pumpelectrodes, each having both an electronic conductive material and anionic conducting material, each of said first and second pump electrodesbeing separate from said sensing and counter electrodes and situated onopposite sides of and in contact with said second protonic conductiveelectrolyte membrane; said first porous pump electrode being exposed toa chamber sealed off from the ambient atmosphere; said second porouspump electrode being separated from said counter electrode by aperforated support structure composed of an electrical conductingmaterial, both said second porous pump electrode and said counterelectrode being in contact with said perforated support structure; meansfor electrical measurement in electrical contact with said sensingelectrode and perforated support structure; means for applying a DCpower across said second protonic electrolyte membrane in electricalcontact with said first pump electrode and said perforated supportstructure; whereby the gas is transported away from the counterelectrode when the means for applying a DC power across said secondprotonic electrolyte membrane applies a DC power across said secondprotonic electrolyte membrane; and whereby, in a positive ambientconcentration of said gas, said electrical measurement means detects iscapable of detecting changes in said electrical characteristic.
 48. Theelectrochemical gas sensor as defined in claim 47, wherein the sensingand counter electrodes have a diameter in a range of 1 mm-15 mm, and theprotonic conductive electrolyte membrane has a thickness in a range of0.1 mm-1 mm.
 49. The electrochemical gas sensor as defined in claim 48,wherein the sensing and electrodes have a diameter of about 10 mm, andthe protonic conductive electrolyte membrane has a thickness of about0.17 mm.
 50. The electrochemical gas sensor as defined in claim 47,wherein the electronic and ionic conducting materials of said sensingand counter electrodes are a proton-electron mixed conductive materialhaving 10-50 wt% of a proton conductor material and 50-90 wt% of a firstand a second electrical conductor materials.
 51. The electrochemical gassensor as defined in claim 50, wherein the proton conductor material forboth the sensing and counter electrodes is a copolymer having atetrafluorethylene backbone with a side chain of perfluorinated monomerscontaining at least one of a sulfonic acid group or a carboxylic acidgroup.
 52. The electrochemical gas sensor as defined in claim 50,wherein one of the first and second electrical conductor materials forthe sensing electrode is 50-99 wt% of carbon black, and the other of thefirst and second electrical conductor materials for the sensingelectrode is 1-50 wt% of platinum.
 53. The electrochemical gas sensor asdefined in claim 50, wherein one of the first and second electricalconductor materials for the counter electrode is 50-99 wt% of carbonblack, and the other of the first and second electrical conductormaterials for the counter-reference electrode is 1-50 wt% of Ru oxide.54. The electrochemical gas sensor as defined in claim 47, wherein theelectrochemical gas sensor is adapted to detect CO.
 55. Theelectrochemical gas sensor as defined in claim 47, wherein theelectrochemical gas sensor is adapted to detect hydrogen.
 56. Theelectrochemical gas sensor as defined in claim 47, wherein theelectrochemical gas sensor is adapted to detect NO_(x).
 57. Theelectrochemical gas sensor as defined in claim 47, wherein theelectrochemical gas sensor is adapted to detect H₂O vapor.
 58. Theelectrochemical gas sensor as defined in claim 47, wherein theelectrochemical gas sensor is adapted to detect H₂S.
 59. Anelectrochemical gas sensor for quantitative measurement of a gas in anambient atmosphere comprising: a porous mixed ionic-electronicconductive sensing electrode having both an electronic conductingmaterial and an ionic conducting material and being exposed to theambient atmosphere; a porous mixed ionic-electronic conductive referenceelectrode having both an electronic conducting material and an ionicconducting material; a porous mixed ionic-conductive counter electrodehaving both an electrical conducting material and an ionic conductingmaterial, and being separate from both said sensing and referenceelectrodes; a protonic conductive electrolyte membrane, having top andbottom sides, said top side of said protonic conductive membrane beingin contact with the counter electrode and the reference electrode, thebottom side of said protonic conductive membrane being in contact withthe sensing electrode; the sensing electrode being capable of reactingwith the gas to produce a change in an electrical characteristic betweenthe sensing electrode and the counter electrode; means for electricalmeasurement in electrical contact between the sensing electrode and thecounter electrode; means for applying a DC power across said protonicelectrolyte membrane in electrical contact between the sensing electrodeand said reference electrode; whereby the gas is transported away fromthe reference electrode when the means for applying a DC power acrosssaid protonic electrolyte membrane applies a DC power across saidprotonic electrolyte membrane; and whereby, in a positive ambientconcentration of said gas, said electrical measurement means detects iscapable of detecting changes in said electrical characteristic.
 60. Theelectrochemical gas sensor as defined in claim 59, wherein said sensing,count and reference electrodes comprise carbon.
 61. The electrochemicalgas sensor as defined in claim 59, wherein said sensing, count andreference electrodes comprise noble metals.
 62. The electrochemical gassensor as defined in claim 59, wherein said sensing, counter andreference electrodes comprise conductive metal oxides.
 63. Theelectrochemical gas sensor as defined in claim 59, wherein the protonicconductive electrolyte membrane is substantially comprised of a solid,perfluorinated, ion-exchange polymer.
 64. The electrochemical gas sensoras defined in claim 59, wherein the protonic conductive electrolytemembrane is a metal oxide protonic conductor electrolyte membrane. 65.The electrochemical gas sensor as defined in claim 59, wherein theelectrochemical gas sensor is adapted to detect CO.
 66. Theelectrochemical gas sensor as defined in claim 59, wherein theelectrochemical gas sensor is adapted to detect NO_(x).
 67. Theelectrochemical gas sensor as defined in claim 59, wherein theelectrochemical gas sensor is adapted to detect hydrogen.
 68. Theelectrochemical gas sensor as defined in claim 59, wherein theelectrochemical gas sensor is adapted to detect H₂S.
 69. Theelectrochemical gas sensor as defined in claim 59, wherein theelectrochemical gas sensor is adapted to detect H₂O vapor.
 70. Theelectrochemical gas sensor as defined in claim 59, wherein the sensing,counter and reference electrodes have a diameter of about 10 mm, and theprotonic conductive electrolyte membrane has a thickness of about 0.17mm.
 71. The electrochemical gas sensor as defined in claim 59, whereinthe electronic and ionic conducting materials of said sensing, counterand reference electrodes are a proton-electron mixed conductive materialhaving 10-50 wt% of a proton conductor material and 50-90 wt% of a firstand second electrical conductor materials.
 72. The electrochemical gassensor as defined in claim 71, wherein the proton conductor material forboth the sensing, counter and reference electrodes is a copolymer havinga tetrafluorethylene backbone with a side chain of perfluorinatedmonomers containing at least one of a sulfonic acid group or acarboxylic acid group.
 73. The electrochemical gas sensor as defined inclaim 71, wherein one of the first and second electrical conductormaterials for the sensing electrode is 50-99 wt% of carbon black, andthe other of the first and second electrical conductor materials for thesensing electrode is 1-50 wt% of platinum.
 74. The electrochemical gassensor as defined in claim 71, wherein one of the first and secondelectrical conductor materials for the counter and reference electrodesis 50-99 wt% of carbon black, and the other of the first and secondelectrical conductor materials for the counter and reference electrodesis 1-50 wt% of Ru oxide.
 75. The electrochemical gas sensor as definedin claim 1, wherein the sensing and the counter electrodes each have afirst side opposite a second side, and wherein the ionic and electronicconducting materials are continuous from the first side to the oppositesecond side within each of the sensing and counter electrodes.
 76. Theelectrochemical gas sensor as defined in claim 30, wherein the sensingand the counter electrodes each have a first side opposite a secondside, and wherein the ionic and electronic conducting materials arecontinuous from the first side to the opposite second side within eachof the sensing and counter electrodes.
 77. The electrochemical gassensor as defined in claim 47, wherein the sensing, counter, firstpumping, and second pumping electrodes each have a first side opposite asecond side, and wherein the ionic and electronic conducting materialsare continuous from the first side to the opposite second side withineach of the sensing, counter, first pumping, and second pumpingelectrodes.
 78. The electrochemical gas sensor as defined in claim 59,wherein the sensing, counter, and reference electrodes each have a firstside opposite a second side, and wherein the ionic and electronicconducting materials are continuous from the first side to the oppositesecond side within each of the sensing, counter, and referenceelectrodes.
 79. An electrochemical gas sensor for quantitativemeasurement of a gas in an ambient atmosphere comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm; the sensing electrode reacting with the gas to produce achange in an electrical characteristic between the sensing electrode andthe counter electrode in the absence of an applied voltage to thesensing electrode; means for electrical measurement; said sensing andcounter electrodes each having a diameter in the range of approximately1 mm to 15 mm, and being electrically connected to said electricalmeasurement means; whereby, in a positive ambient concentration of saidgas, said electrical measurement means detects changes in saidelectrical characteristic.
 80. A two-electrode electrochemical gassensor for quantitative measurement of a gas in an ambient atmospherecomprising: a porous mixed ionic-electronic conductive sensing electrodehaving both an electronic conducting material and an ionic conductingmaterial; a porous mixed ionic-electronic conductive counter electrodehaving both an electronic conducting material and an ionic conductingmaterial; wherein the electronic and ionic conducting materials of saidsensing and counter electrodes are proton-electron mixed conductivematerials having 10-50 wt % of a proton conductor material and 50-90 wt% of a first and a second electrical conductor materials; a firstprotonic conductive electrolyte membrane in between and in contact withthe sensing and counter electrodes, and having a thickness in the rangeof approximately 0.1 mm to 1 mm, the sensing electrode and the counterelectrode being the only two electrodes in contact with the firstprotonic conductive electrolyte membrane; the sensing electrode reactingwith the gas to produce a change in an electrical characteristic betweenthe sensing electrode and the counter electrode in the absence of anapplied voltage to the sensing electrode; means for electricalmeasurement; said sensing and counter electrodes each having a diameterin the range of approximately 1 mm to 15 mm, and being electricallyconnected to said electrical measurement means; whereby, in a positiveambient concentration of said gas, said electrical measurement meansdetects changes in said electrical characteristic.
 81. Anelectrochemical gas sensor for quantitative measurement of a gas in anambient atmosphere comprising: a porous mixed ionic-electronicconductive sensing electrode having both an electronic conductingmaterial and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm, the sensing electrode and the counter electrode being onopposite sides of the first protonic conductive electrolyte membrane;the sensing electrode reacting with the gas to produce a change in anelectrical characteristic between the sensing electrode and the counterelectrode; means for electrical measurement; said sensing and counterelectrodes each having a diameter in the range of approximately 1 mm to15 mm, and being electrically connected to said electrical measurementmeans; whereby, in a positive ambient concentration of said gas, saidelectrical measurement means detects changes in said electricalcharacteristic; and wherein the sensing electrode reacts with the gas inthe absence of an applied voltage to the sensing electrode.
 82. Anelectrochemical gas sensor for quantitative measurement of a gas in anambient atmosphere at room temperature, comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm; the sensing electrode reacting with the gas at roomtemperature to produce a change in an electrical characteristic betweenthe sensing electrode and the counter electrode in the absence of anapplied voltage to the sensing electrode; said sensing and counterelectrodes each having a diameter in the range of approximately 1 mm to15 mm, and being electrically connected to said electrical measurementmeans; and means for electrical measurement; whereby, in a positiveambient concentration of said gas, said electrical measurement meansdetects changes in said electrical characteristic.
 83. A two-electrodeelectrochemical gas sensor for quantitative measurement of a gas in anambient atmosphere at room temperature comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm, the sensing electrode and the counter electrode beingthe only two electrodes in contact with the first protonic conductiveelectrolyte membrane; the sensing electrode reacting with the gas atroom temperature to produce a change in an electrical characteristicbetween the sensing electrode and the counter electrode in the absenceof an applied voltage to the sensing electrode; said sensing and counterelectrodes each having a diameter in the range of approximately 1 mm to15 mm, and being electrically connected to said electrical measurementmeans; and means for electrical measurement; whereby, in a positiveambient concentration of said gas at room temperature, said electricalmeasurement means detects changes in said electrical characteristic. 84.An electrochemical gas sensor for quantitative measurement of a gas inan ambient atmosphere at room temperature, comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material; a porous mixedionic-electronic conductive counter electrode having both an electronicconducting material and an ionic conducting material; and wherein theelectronic and ionic conducting materials of said sensing and counterelectrodes are proton-electron mixed conductive materials having 10-50wt % of a proton conductor material and 50-90 wt % of a first and asecond electrical conductor materials; a first protonic conductiveelectrolyte membrane in between and in contact with the sensing andcounter electrodes, and having a thickness in the range of approximately0.1 mm to 1 mm, the sensing electrode and the counter electrode being onopposite sides of the first protonic conductive electrolyte membrane;the sensing electrode reacting with the gas at room temperature toproduce a change in an electrical characteristic between the sensingelectrode and the counter electrode; said sensing and counter electrodeseach having a diameter in the range of approximately 1 mm to 15 mm, andbeing electrically connected to said electrical measurement means; andmeans for electrical measurement; whereby, in a positive ambientconcentration of said gas at room temperature, said electricalmeasurement means detects changes in said electrical characteristic; andwherein the sensing electrode is capable of reacting with the gas atroom temperature in the absence of an applied voltage to the sensingelectrode.
 85. A non-biased electrochemical gas sensor for quantitativemeasurement of a gas in an ambient atmosphere at room temperaturecomprising: a porous mixed ionic-electronic conductive sensing electrodehaving both an electronic conducting material and an ionic conductingmaterial; a porous mixed ionic-electronic conductive counter electrodehaving both an electronic conducting material and an ionic conductingmaterial; wherein the electronic and ionic conducting materials of saidsensing and counter electrodes are proton-electron mixed conductivematerials having 10-50 wt % of a proton conductor material and 50-90 wt% of a first and a second electrical conductor materials; a firstprotonic conductive electrolyte membrane in between and in contact withthe sensing and counter electrodes, and having a thickness in the rangeof approximately 0.1 mm to 1 mm; the sensing electrode reacting with thegas at room temperature to produce a change in an electricalcharacteristic between the sensing electrode and the counter electrode;said sensing and counter electrodes each having a diameter in the rangeof approximately 1 mm to 15 mm, and being electrically connected to saidelectrical measurement means; and means for electrical measurement;whereby, in a positive ambient concentration of said gas at roomtemperature, said electrical measurement means detects changes in saidelectrical characteristic in the absence of any biasing voltage.
 86. Thenon-biased electrochemical gas sensor of claim 85 in which the sensingelectrode and the counter electrode are the only two electrodes incontact with the first protonic conductive electrolyte membrane.
 87. Thenon-biased electrochemical gas sensor of claim 86 in which the sensingelectrode is capable of reacting with carbon monoxide at roomtemperature to produce a change in electrical characteristic between thesensing electrode and the counter electrode in the absence of an appliedvoltage to the sensing electrode.
 88. A two-electrode electrochemicalgas sensor for quantitative measurement of a carbon monoxide gas in anambient atmosphere at room temperature comprising: a porous mixedionic-electronic conductive sensing electrode having both an electronicconducting material and an ionic conducting material, the sensingelectrode includes platinum, carbon and a copolymer having atetrafluorethylene backbone with a side chain of perfluorinated monomerscontaining a sulfonic acid group; a porous mixed ionic-electronicconductive counter electrode having both an electronic conductingmaterial and an ionic conducting material, the counter electrodeincludes platinum, carbon and a copolymer having a tetrafluorethylenebackbone with a side chain of perfluorinated monomers containing asulfonic acid group; wherein the electronic and ionic conductingmaterials of said sensing and counter electrodes are proton-electronmixed conductive materials having 10-50 wt % of a proton conductormaterial and 50-90 wt % of a first and a second electrical conductormaterials; a first protonic conductive solid electrolyte membrane inbetween and in contact with the sensing and counter electrodes, andhaving a thickness in the range of approximately 0.1 mm to 1 mm, theprotonic conductive solid electrolyte membrane being substantiallycomprised of a solid, perfluorinated, ion-exchange polymer and beingapproximately 0.17 mm thick; the sensing electrode reacting with thecarbon monoxide gas to produce a change in an electrical characteristicbetween the sensing electrode and the counter electrode, the sensingelectrode and the counter electrode being the only two electrodes incontact with the first protonic conductive electrolyte membrane and thesensing electrode and the counter electrode being on opposite sides ofthe first protonic conductive electrolyte membrane; means for electricalmeasurement electrically connected to said sensing and counterelectrodes; said sensing and counter electrodes each having a diameterin the range of approximately 1 mm to 15 mm, and being electricallyconnected to said electrical measurement means, the sensing electrodebeing 15 mm in diameter and the counter electrode being approximately 15mm in diameter; whereby said electrical measurement means detectschanges in said electrical characteristic in a positive ambientatmosphere concentration of said gas at room temperature.